The present invention relates generally to communication systems and methods and, more particularly, to systems and methods for handling frame discard indication within cell-based data communication networks, i.e., indications at the interface of data communication networks with cell-based networks, such as asynchronous transfer mode (ATM) networks.
ATM technology has played a central role in the evolution of workgroup and enterprise networks. It has the capability to provide scalable bandwidth at commercially attractive price and performance guarantees, which have facilitated new classes of data applications, such as multimedia.
ATM networks are formed from ATM switches interconnected by ATM links or interfaces, an example of which is shown in FIG. 1. Therein, an ATM network 10 includes two ATM switches 12 and 14. ATM switch 12 supports data communication with two end user (edge) devices 16 and 18, e.g., computers, while ATM switch 14 provides data communication between the network 10 and router 20. Those skilled in the art will appreciate that many ATM networks will contain substantially more ATM switches and user devices than those shown in FIG. 1 and that such ATM networks will themselves be connected to other ATM networks, e.g., network 22 in FIG. 1. Typically there will also be a network management system (not shown in FIG. 1) that is connected to all of the system nodes for monitoring purposes.
Signaling between the nodes in ATM network 10 is performed as described in various standardized interface specifications. ATM networks employ two different types of interfaces: user-network interfaces (UNI) and network-network interfaces (NNI) to support this signaling. Generally speaking, UNI interfaces are used for communication between an ATM switch and an end user device or system (a personal computer, a router, a host, etc.), while NNI interfaces are used for communication between ATM switches. Of particular interest for understanding the present invention are the UNI signaling specifications, which are described in the document entitled “ATM User-Network Interface (UNI) Signaling Specification”, Version 4.1, April 2002, promulgated by the ATM Forum Technical Committee. This document (hereafter referred to as “UNI 4.1 ”) is available, for example, from the ATM Worldwide Headquarters, Presidio of San Francisco, 572B Ruger Street, San Francisco, Calif. 94129 and the contents thereof are expressly incorporated herein by reference.
Even more specifically, the present invention is related to the UNI signaling associated with frame discard. Table A13.1 of UNI 4.1 (reproduced herein as FIG. 2) describes how an ATM node interprets a received frame discard indicator based on the signaled ATM Adaptation Layer (AAL) type, which provides the interface between the user layer and the ATM layer for applications with similar requirements. Different AAL types are defined to support different types of traffic. Those skilled in the art will appreciate that the various types of AAL (e.g., AAL 0, AAL 1, AAL 2, AAL 3 /4 and AAL 5) are employed to enable variably sized data packets to be processed and transported via fixed sized ATM cells. AAL5 includes an “end of message” to indicate to the ATM layer that all the cells that correspond to a user frame have been processed. Thus, for AAL5, the ATM layer is aware of the boundaries of the user data frame. In the event of congestion, if frame discard allowed has been signaled by the user, the ATM layer can intelligently discard all the cells that correspond to the entire data frame, rather than less efficiently discarding cell by cell. Note that for all AAL types other than AAL 5, when frame discard is allowed the ATM layer is not aware of the frame boundaries. Thus, it may end up randomly discarding cells from different frames. According to Table A13.1 of UNI 4.1, this is the condition that occurs when the signaled frame discard indicator is set to 1 and the AAL type is different from AAL5. Flows which lack the frame delimiter required to indicate frame boundaries are referred to herein as “non-delineable flows”.
Discarding cells without knowing the frame boundaries can cause problems in ATM networks. For example, a user can initiate an ATM connection using AAL 5 with the frame discard feature allowed and later switch to another AAL type without adding the required frame delimiter to the appropriate ATM cell header. When congested cells are discarded at random (because there is no concept of end of frame in the other AAL types), the cells may belong to different frames, resulting in the loss of data from several frames. The user application may then initiate retransmission of the lost data, thereby exacerbating the congestion condition in the network element. Furthermore, any network interface specifications that do not provide mechanisms for handling non-delineable flows (such as UNI 4.0, UNI 4.1 or any other specifications) may cause the throughput for non-delineable flows to drop significantly if the buffer management mechanism of the network element does not take that condition into account. Identifying and correcting this problem after it occurs poses significant challenges for a network operator given the complexity of ATM systems
Accordingly, it would be desirable to provide systems and methods for handling non-delineable flows associated with frame discard in ATM systems.